Physics Solutions Manual

Chapter 4 continued
3 Evaluate the Answer
• Are the units correct? N is the correct
unit for a tension, since it is a force.
• Does the sign make sense? The positive
sign indicates that the tension is
pulling upwards.
• Is the magnitude realistic? We would
expect the magnitude to be on the
same order as the block’s weight.
98. Think Critically Because of your physics
knowledge, you are serving as a scientific
consultant for a new science-fiction TV
series about space exploration. In episode 3,
the heroine, Misty Moonglow, has been
asked to be the first person to ride in a new
interplanetary transport for use in our solar
system. She wants to be sure that the transport
actually takes her to the planet she is
supposed to be going to, so she needs to
take a testing device along with her to
measure the force of gravity when she
arrives. The script writers don’t want her to
just drop an object, because it will be hard
to depict different accelerations of falling
objects on TV. They think they’d like something
involving a scale. It is your job to
design a quick experiment Misty can conduct
involving a scale to determine which
planet in our solar system she has arrived
on. Describe the experiment and include
what the results would be for Pluto
(g 0.30 m/s 2 ), which is where she is supposed
to go, and Mercury (g 3.70 m/s 2 ),
which is where she actually ends up.
Answers will vary. Here is one possible
answer: She should take a known mass,
say 5.00-kg, with her and place it on the
scale. Since the gravitational force
depends upon the local acceleration
due to gravity, the scale will read a different
number of newtons, depending
on which planet she is on. The following
analysis shows how to figure out what
the scale would read on a given planet:
Identify the mass as the system and
upward as positive.
F net F scale on mass F g ma 0
F scale on mass F g
F scale on mass mg
Pluto: F scale on mass
(5.00 kg)(0.30 m/s 2 )
1.5 N
Mercury: F scale on mass
(5.00 kg)(3.7 m/s 2 )
19 N
99. Apply Concepts Develop a CBL lab, using
a motion detector, that graphs the distance
a free-falling object moves over equal intervals
of time. Also graph velocity versus time.
Compare and contrast your graphs. Using
your velocity graph, determine the acceleration.
Does it equal g?
Student labs will vary with equipment
available and designs. p-t graphs and
v-t graphs should reflect uniform acceleration.
The acceleration should be
close to g.
Writing in Physics
page 116
100. Research Newton’s contributions to physics
and write a one-page summary. Do you think
his three laws of motion were his greatest
accomplishments? Explain why or why not.
Answers will vary. Newton’s contributions
should include his work on light
and color, telescopes, astronomy, laws
of motion, gravity, and perhaps
calculus. One argument in favor of his
three laws of motion being his greatest
accomplishments is that mechanics is
based on the foundation of these laws.
His advances in the understanding of
the concept of gravity may be suggested
as his greatest accomplishment
instead of his three laws of motion.
84 Solutions Manual Physics: Principles and Problems
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